Broadband wireless access (BWA) is an area of much growth in recent times. As users become increasingly mobile, one of the new areas of providing this BWA is on mobile platforms such as on railways, highways and similar routes of passage. Mobile Distributed Antenna Systems-(MDAS) can be deployed to provide such continuous wireless coverage for BWA over these passages. MDAS Remote Antenna Units (RAUs) are placed at sites with poor signal strength to improve coverage. These RAUs are connected back to the central head-end office via fiber where the base station resides. These RAUs essentially replicate the signal generated by the base station in the downlink direction as well as replicate the signal generated by the mobile station in the uplink direction. These replicated signals are transmitted over fiber in the appropriate uplink and downlink direction. Thereby, the MDAS concept is a fiber based one-to-many (and many-to-one) repeater system.
However, this MDAS concept can introduce severe multipath effects that will compromise data integrity. This happens when the receiver receives multiple copies of the same signal at different times transmitted by different RAUs; such delays arise from different fiber and wireless distances. The resulting echo of previous data will create interference at the receiver for the current datagram and the echo of the current data will interfere with a subsequent datagram. Loss of data and/or loss of acknowledgement signals can result such that overall data rate is reduced.
MDAS systems have high wireless transmission power requirements as coverage areas are typically large. High power consumption is associated with such high power wireless transmission systems. For extensively deployed DAS for mobile BWAs, many RAUs are needed to ensure sufficiently high signal-to-noise ratio to support the high data rates prescribed in 4th generation BWA. Thus, power consumption is substantial for many RAUs. Therefore, there is a need to reduce overall DAS system power consumption into order to reduce operating costs.
As the number of RAUs within a DAS system increases, the increasing number of active uplink RAU circuits contribute to noise at the receiver at the head-end. This increases the noise floor for the system and thus reduces overall performance and receiver sensitivity suffers. Thus overall noise floor of the system increases with increasing number of active RAUs. In a larger DAS system, this increase in overall noise floor can reduce the sensitivity of the receiver and reduce the coverage of the RAU.